This study aims at investigating mechanisms of directional selectivity in cortical simple cells in the cat and monkey, by comparing and testing a number of competing models that have been proposed for motion detection in biological systems. Four classes of simple cells (Schiller's S-1, S-2, S-3, S-6) have been selected for our study. Using available physiological data and detailed computer simulation we have derived specific predictions from the different models, and intend to compare them against physiological recordings, using stimuli designed to optimally separate among models. The comparisons are designed to separate between main families of models before testing specific models in greater detail. The contribution of this study will be in clarifying properties of directionally selective mechanisms, investigating the structure of cortical simple cells and possible hierarchies among them, and suggesting methods that may be applicable to the computerized analysis of time-varying imagery.